1. Field of the Invention
The present invention relates to a display device and particularly to a driver circuit for displaying images in the display device. The present invention also relates to an electronic appliance including the display device.
2. Description of the Related Art
In recent years, the popularity of display devices such as liquid crystal display devices and electroluminescent (EL) display devices has increased rapidly because such display devices can be lightweight and have a large-area screen as compared with CRT display devices.
Driving methods of display devices include a passive matrix method and an active matrix method. An active matrix method has been employed for display portions of television receivers, cellular phones, and the like because an active matrix method can achieve lower power consumption, higher definition, enlargement of substrates, and the like as compared with a passive matrix method.
Panels of an active matrix method have come to be larger in size and to have higher definition, and are required to achieve higher performance of driver circuits for controlling driving of pixel portions. Therefore, a technique has been employed in which a driver circuit is mounted by a COG (chip on glass) method (see Reference 1: Japanese Published Patent Application No. 2003-255386) or by a TAB (tape automated bonding) method. FIG. 19A shows a display device employing a COG method.
In FIG. 19A, a pixel portion in which pixels are arranged in matrix is formed over a substrate 1900 having an insulating surface and a counter substrate 1901 is formed so as to cover the pixel portion. The pixels are arranged in matrix in such a manner that each pixel is disposed at an intersection of a scanning line 1903 extending from each scanning line side driver IC 1902 and a signal line 1905 extending from each signal line side driver IC 1904. Each pixel in the pixel portion is provided with a switching element and a pixel electrode layer connected to the switching element. A typical example of the switching element is a thin film transistor, and a thin film transistor has its gate connected to the scanning line 1903 and its source or drain connected to the signal line 1905. Moreover, the scanning line side driver ICs 1902 and the signal line side driver ICs 1904 are each connected to an FPC (flexible printed circuit) 1906. FIG. 19A also shows a cross section of the display device along a dotted line A-B.
As another method of mounting high-performance driver circuits, the following technique is given as disclosed in Reference 2 (Japanese Published Patent Application No. H11-160734): driver circuits are formed using thin film transistors (TFTs; also simply called transistors) manufactured with use of a non-single-crystal semiconductor material whose crystallinity has been enhanced by laser irradiation over a substrate made of glass or the like; the substrate having the driver circuits is divided into strips (stick forms); and then the stick-form driver circuits are mounted on a display device. FIG. 19B shows a display device of a driving method which uses stick-form driver circuits (this driving method is hereinafter referred to as a stick method).
In FIG. 19B, a pixel portion in which pixels are arranged in matrix is formed over a substrate 1950 having an insulating surface and a counter substrate 1951 is formed so as to cover the pixel portion. The pixels are arranged in matrix in such a manner that each pixel is disposed at an intersection of a scanning line 1953 extending from a scanning line side stick driver 1952 and a signal line 1955 extending from a signal line side stick driver 1954. Each pixel in the pixel portion is provided with a switching element and a pixel electrode layer connected to the switching element. A typical example of the switching element is a thin film transistor, and a thin film transistor has its gate connected to the scanning line 1953 and its source or drain connected to the signal line 1955. Moreover, the scanning line side stick driver 1952 and the signal line side stick driver 1954 are each connected to an FPC (flexible printed circuit) 1956. FIG. 19B also shows a cross section of the display device along a dotted line A-B.
The display device employing a COG method shown in FIG. 19A has the scanning line side driver ICs 1902 and the signal line side driver ICs 1904 provided apart from each other at a predetermined interval in accordance with the size of the display device. The integration degree of the scanning line side driver ICs 1902 and the signal line side driver ICs 1904 is high; however, the pixel portion can operate without troubles even though these ICs are provided apart from each other. In contrast to this, it is necessary to lead wirings to the pixel portion when the scanning lines 1903 and the signal lines 1905 extend. Therefore, a frame region 1921 (a peripheral region of the pixel portion) of the display device needs to be very wide. In the display device, the increase in width of the frame region is disadvantageous in that the small size, which is an advantage of liquid crystal display devices or EL display devices, is not achieved.
In the display device of the stick method shown in FIG. 19B, the scanning line side stick driver 1952 and the signal line side stick driver 1954 are provided in accordance with the size of the display device. As for the size, the scanning line side stick driver 1952 and the signal line side stick driver 1954 can be formed in accordance with the size of the pixel portion because these ICs are obtained by dividing glass substrates. As for their characteristics, on the other hand, since the driver circuits are formed using thin film transistors with use of a non-single-crystal semiconductor material whose crystallinity has been enhanced by laser irradiation, the characteristics of the thin film transistors in the driver circuits, such as threshold voltage, vary in some cases due to laser fringes caused at the time of the laser irradiation. As a result, the scanning line side stick driver 1952 and the signal line side stick driver 1954 formed with the use of the non-single-crystal semiconductor material whose crystallinity has been enhanced by the laser irradiation has a possibility of causing an operation error.